Rework host authentication for SFTP storage backends

[orangelynx]

duplicacy/src/duplicacy_storage.go

Lines 165 to 211 in 0a794e6

	func checkHostKey(hostname string, remote net.Addr, key ssh.PublicKey) error {
	if preferencePath == "" {
	return fmt.Errorf("Can't verify SSH host since the preference path is not set")
	}
	hostFile := path.Join(preferencePath, "known_hosts")
	file, err := os.OpenFile(hostFile, os.O_RDWR|os.O_CREATE, 0600)
	if err != nil {
	return err
	}
	defer file.Close()
	content, err := ioutil.ReadAll(file)
	if err != nil {
	return err
	}
	lineRegex := regexp.MustCompile(`^([^\s]+)\s+(.+)`)
	keyString := string(ssh.MarshalAuthorizedKey(key))
	keyString = strings.Replace(keyString, "\n", "", -1)
	remoteAddress := remote.String()
	if strings.HasSuffix(remoteAddress, ":22") {
	remoteAddress = remoteAddress[:len(remoteAddress)-len(":22")]
	}
	for i, line := range strings.Split(string(content), "\n") {
	matched := lineRegex.FindStringSubmatch(line)
	if matched == nil {
	continue
	}
	if matched[1] == remote.String() {
	if keyString != matched[2] {
	LOG_WARN("HOSTKEY_OLD", "The existing key for '%s' is %s (file %s, line %d)",
	remote.String(), matched[2], hostFile, i)
	LOG_WARN("HOSTKEY_NEW", "The new key is '%s'", keyString)
	return fmt.Errorf("The host key for '%s' has changed", remote.String())
	} else {
	return nil
	}
	}
	}
	file.Write([]byte(remote.String() + " " + keyString + "\n"))
	return nil
	}

Just something I stumbled across trying to understand how duplicacy verifies the host it is connecting to.

1. This code is not fully compatible with the standard definition of the SSH known_hosts file syntax [1]. Perhaps it's not meant to be, but then the name of the file is misleading. While the code appears to be able to account for markers, it does not seem to be able to account for the brackets around the host name, in case a non-standard port is provided. Also if the standard port :22 is used, it is not enforced that it is omitted in the config file.

2. I'd argue that duplicacy should not automatically connect to hosts for which it has never seen a fingerprint before, without explicit user consent. Right now, a MITM could be happening during the first use of duplicacy, leading to exposed data.

3. Maybe consider taking into account ~/.ssh/known_hosts as well in addition to .duplicacy/known_hosts?

[1] https://man.openbsd.org/sshd#SSH_KNOWN_HOSTS_FILE_FORMAT